U.S. patent number 8,169,147 [Application Number 12/561,483] was granted by the patent office on 2012-05-01 for circuit for vehicle lighting.
This patent grant is currently assigned to O2Micro, Inc.. Invention is credited to Cheng-Wei Hsu, ShengTai Lee, Yung Lin Lin, Da Liu.
United States Patent |
8,169,147 |
Hsu , et al. |
May 1, 2012 |
Circuit for vehicle lighting
Abstract
A circuit for driving a vehicle lamp includes a current path
coupled between a power line and ground, and a monitoring unit
coupled to the power line. The current path includes a dummy load.
The monitoring unit can monitor a testing signal applied to the
power line. The testing signal can test whether the vehicle lamp
operates properly. The monitoring unit can conduct the current path
to enable a current to flow through the dummy load to ground to
decrease a total resistance of the circuit if the testing signal is
detected.
Inventors: |
Hsu; Cheng-Wei (Taipei,
TW), Liu; Da (Milpitas, CA), Lee; ShengTai
(Taipei, TW), Lin; Yung Lin (Palo Alto, CA) |
Assignee: |
O2Micro, Inc. (Santa Clara,
CA)
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Family
ID: |
43729817 |
Appl.
No.: |
12/561,483 |
Filed: |
September 17, 2009 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20110062869 A1 |
Mar 17, 2011 |
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Current U.S.
Class: |
315/77; 324/414;
315/82 |
Current CPC
Class: |
H05B
45/50 (20200101); B60Q 11/005 (20130101); H05B
45/58 (20200101); Y02B 20/383 (20130101); Y02B
20/30 (20130101) |
Current International
Class: |
B60Q
1/00 (20060101); G01R 31/00 (20060101) |
Field of
Search: |
;315/77,82,291,294,297,301,307,313 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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101553063 |
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Oct 2009 |
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CN |
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101754530 |
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Jun 2010 |
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CN |
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Primary Examiner: Chang; Daniel D
Claims
What is claimed is:
1. A circuit for driving a vehicle lamp, comprising: a current path
coupled between a power line and ground and comprising a dummy
load; and a monitoring unit coupled to said power line and operable
for monitoring a testing signal applied to said power line, wherein
said testing signal is operable for testing whether said vehicle
lamp operates properly, and wherein said monitoring unit is
operable for conducting said current path to enable a current to
flow through said dummy load to ground to decrease a total
resistance of said circuit if said testing signal is detected.
2. The circuit of claim 1, further comprising: a DC/DC converter
coupled to said power line and operable for providing regulated
power to said vehicle lamp; a current sensor coupled to said
vehicle lamp and operable for providing a sensing signal indicative
of a current flowing through said vehicle lamp; and a controller
coupled to said current sensor and operable for controlling said
DC/DC converter based on said sensing signal, and operable for
determining a conductance status of said current path based on said
sensing signal.
3. The circuit of claim 2, wherein said controller is operable for
cutting off said current path if said vehicle lamp is in an open
circuit condition.
4. The circuit of claim 2, wherein said controller is operable for
cutting off said current path if said vehicle lamp is in a short
circuit condition.
5. The circuit of claim 1, wherein said dummy load comprises a
resistor.
6. The circuit of claim 1, wherein said monitoring unit comprises a
capacitor, wherein a voltage across said capacitor increases above
a threshold if said testing signal is applied to said power line,
and wherein said capacitor turns on a switch in said current path
if said voltage across said capacitor is above said threshold.
7. The circuit of claim 6, wherein said voltage across said
capacitor is below said threshold if said testing signal is absent
from said power line, and wherein said switch is turned off if said
voltage across said capacitor is below said threshold.
8. The circuit of claim 1, wherein said monitoring unit comprises a
first capacitor coupled to said power line and for passing through
said test signal and for isolating DC power on said power line.
9. The circuit of claim 8, wherein said monitoring unit further
comprises: a first diode with a cathode coupled to said first
capacitor and an anode coupled to ground; and a second diode with
an anode coupled to said first capacitor and a cathode coupled to
ground through a second capacitor, wherein said current path
comprises a switch, and wherein a conductance status of said switch
is determined by a voltage across said second capacitor.
10. A light assembly, comprising: a light source; and a circuit
coupled to a power line for driving said light source, comprising:
a first capacitor coupled to said power line, wherein a voltage
across said first capacitor increases above a threshold if a
testing signal for testing whether said light source operates
properly is applied to said power line; a current path coupled
between said power line and ground, wherein said current path is
conducted to decrease a total resistance of said circuit if said
voltage across said first capacitor is above said threshold.
11. The light assembly of claim 10, further comprising: a DC/DC
converter coupled to said power line and operable for providing
regulated power to said light source; a current sensor coupled to
said light source and operable for providing a sensing signal
indicative of a current flowing through said light source; and a
controller coupled to said current sensor and operable for
controlling said DC/DC converter based on said sensing signal, and
operable for determining a conductance status of said current path
based on said sensing signal.
12. The light assembly of claim 11, wherein said controller is
operable for cutting off said current path if said light source is
in an open circuit condition.
13. The light assembly of claim 11, wherein said controller is
operable for cutting off said current path if said light source is
in a short circuit condition.
14. The light assembly of claim 10, wherein said current path
comprises a resistor.
15. The light assembly of claim 10, wherein said light source
comprises a light emitting diode (LED) string.
16. The light assembly of claim 10, wherein said circuit comprises:
a second capacitor coupled to said power line; a first diode with a
cathode coupled to said second capacitor and an anode coupled to
ground; and a second diode with an anode coupled to said second
capacitor and a cathode coupled to ground through said first
capacitor; wherein said current path comprises a switch, and
wherein a conductance status of said switch is determined by a
voltage across said first capacitor.
17. A method for driving a vehicle lamp by a circuit, comprising:
monitoring a testing signal on a power line, wherein said testing
signal tests whether said vehicle lamp operates properly;
conducting a current path between said power line and ground to
decrease a total resistance of said circuit if said testing signal
is detected; and cutting off said current path if said testing
signal is absent.
18. The method of claim 17, further comprising: monitoring a
current flowing through said vehicle lamp; and cutting off said
current path if said vehicle lamp is in an open circuit
condition.
19. The method of claim 17, further comprising: monitoring a
current flowing through said vehicle lamp; and cutting off said
current path if said vehicle lamp is in a short circuit
condition.
20. The method of claim 17, further comprising: converting input
power from said power line to regulated power by a DC/DC converter;
controlling said DC/DC converter based on a current flowing through
said vehicle lamp; and powering said vehicle lamp by said regulated
power.
21. The method of claim 17, further comprising: charging a
capacitor by said testing signal; and turning on a switch in said
current path to conduct said current path if a voltage across said
capacitor is greater than a threshold.
Description
BACKGROUND
In recent years, light sources such as light emitting diodes (LEDs)
have been improved through technological advances in material and
manufacturing processes. The LEDs possess characteristics such as a
relatively high efficiency, a relatively long life, and vivid
colors, and can be used in a variety of industries. One example is
to use the LEDs to replace traditional incandescent bulbs in a
vehicle lamp. Compared with traditional incandescent bulbs, the
LEDs are lighter, compact, long-life, and energy-saving. Moreover,
the response time of the LEDs is faster than that of the
incandescent bulbs.
For some vehicles that are originally designed to be equipped with
incandescent bulbs, there will be a problem if the incandescent
bulbs are directly replaced by LEDs. FIG. 1 illustrates a
conventional circuit 100 for using an incandescent bulb 102 in a
vehicle. The incandescent bulb 102 is powered by a power source
108, e.g., a battery, via a power line 104. Under certain
circumstances, a vehicle may need to perform a self-testing to
examine whether the incandescent bulb 102 is turned on properly. A
micro controlling unit (MCU) in the vehicle (not shown in FIG. 1)
may generate a testing signal (usually a square wave signal) and
apply the testing signal to the power line 104. A detecting circuit
106 monitors the voltage drop across the incandescent bulb 102. If
a waveform of the testing signal has an amplitude greater than a
predetermined level, the waveform can be detected by the detecting
circuit 106. If the incandescent bulb 102 operates properly, the
voltage drop across the incandescent bulb 102 is relatively small
because the resistance of the filament in the incandescent bulb 102
is relatively small. Therefore, the waveform of the testing signal
is not detected by the detecting circuit 106. If the incandescent
bulb 102 is broken down (open circuit condition), the waveform of
the testing signal can be detected by the detecting circuit 106
across the incandescent bulb 102. If the testing signal is detected
by the detecting circuit 106, the detecting circuit 106 can
determine that the incandescent bulb 102 is broken and remind the
driver by turning on an indicator light on the dashboard.
FIG. 2 illustrates a conventional circuit 200 using LEDs to replace
a traditional incandescent bulb in a vehicle. As shown in FIG. 2,
an LED string 202 takes place of the incandescent bulb. The LED
string 202 includes multiple LEDs connected in series. Generally,
the resistance of the LED string 202 is greater than the resistance
of an incandescent bulb. Therefore, when the micro controlling unit
(not shown in FIG. 2) applies the testing signal on the power line
104, a waveform of the testing signal may be detected by the
detecting circuit 106 across the LED string 202, even if the LED
string 202 operates properly. The micro controlling unit may render
an erred judgment. To prevent a false alarm, a dummy load, e.g., a
resistor 204, is coupled to the LED string 202 in parallel. The
resistor 204 can have a relatively small resistance such that the
total resistance of the parallel-connected dummy load 204 and the
LED string 202 is even smaller. By properly choosing the resistance
of the resistor 204, the testing signal is not detected by the
detecting circuit 106 across the LED string 202 such that the false
alarm can be avoided. A drawback of this solution is that the
resistor 204 will constantly consume power and generate heat if the
vehicle lamp is turned on.
SUMMARY
A circuit for driving a vehicle lamp includes a current path
coupled between a power line and ground, and a monitoring unit
coupled to the power line. The current path includes a dummy load.
The monitoring unit can monitor a testing signal applied to the
power line. The testing signal can test whether the vehicle lamp
operates properly. The monitoring unit can conduct the current path
to enable a current to flow through the dummy load to ground to
decrease a total resistance of the circuit if the testing signal is
detected.
BRIEF DESCRIPTION OF THE DRAWINGS
Features and advantages of embodiments of the claimed subject
matter will become apparent as the following detailed description
proceeds, and upon reference to the drawings, wherein like numerals
depict like parts, and in which:
FIG. 1 shows a conventional circuit for using an incandescent bulb
in a vehicle.
FIG. 2 shows a conventional circuit for using LEDs to replace an
incandescent bulb in a vehicle.
FIG. 3 shows a circuit using LEDs to replace an incandescent bulb
in a vehicle, in accordance with one embodiment of the present
invention.
FIG. 4 shows a circuit using LEDs to replace an incandescent bulb
in a vehicle, in accordance with one embodiment of the present
invention.
FIG. 5 illustrates a relationship between a voltage across the
capacitor 410 in FIG. 4 and the testing signal applied on the power
line, in accordance with one embodiment of the present
invention.
FIG. 6 shows a flowchart of a method for powering a light source in
a vehicle, in accordance with one embodiment of the present
invention.
DETAILED DESCRIPTION
Reference will now be made in detail to the embodiments of the
present invention. While the invention will be described in
conjunction with these embodiments, it will be understood that they
are not intended to limit the invention to these embodiments. On
the contrary, the invention is intended to cover alternatives,
modifications and equivalents, which may be included within the
spirit and scope of the invention as defined by the appended
claims.
Furthermore, in the following detailed description of the present
invention, numerous specific details are set forth in order to
provide a thorough understanding of the present invention. However,
it will be recognized by one of ordinary skill in the art that the
present invention may be practiced without these specific details.
In other instances, well known methods, procedures, components, and
circuits have not been described in detail as not to unnecessarily
obscure aspects of the present invention.
FIG. 3 shows a block diagram of a circuit 300 using LEDs to replace
an incandescent bulb in a vehicle, in accordance with one
embodiment of the present invention. In one embodiment, the circuit
300 is integrated in a vehicle light assembly. The circuit 300 is
coupled to a power source 314 through a power line 306. In one
embodiment, the power source 314 can be a battery in a vehicle.
The circuit 300 includes a current path coupled between the power
line 306 and ground. In one embodiment, the current path includes a
dummy load 304 and a switch 318 coupled in series. A monitoring
unit 302 is coupled to the power line 306 and can monitor a testing
signal on the power line 306. The monitoring unit 302 can control
an on/off status of the switch 318 to selectively conduct the
current path. If the monitoring unit 302 detects the testing signal
on the power line 306, the monitoring unit 302 can switch on the
switch 318 to enable a current flowing through the dummy load 304
to ground. As a result, the total resistance of the circuit 300 is
decreased. The circuit 300 can further include a DC/DC converter
310 coupled to the power line 306 for providing regulated power to
a light source, e.g., an LED string 312. A current sensor 316 can
monitor a current flowing through the LED string 312, and can send
a sensing signal indicative of the current flowing through the LED
string 312 to a controller 308. The controller 308 is coupled to
the DC/DC controller 310 and the current sensor 316, and can
control the DC/DC converter 310 based on the sensing signal
provided by the current sensor 316. Thus, the DC/DC converter 310
can provide regulated power to the LED string 312. The controller
308 is also coupled to the switch 318, and can also determine a
conductance status of the current path by controlling the switch
318 based on the current flowing through the LED string 312.
Advantageously, when the vehicle performs a self-testing to examine
whether an incandescent bulb in the light assembly can be properly
turned on, the circuit 300 can prevent a false alarm if the LED
string 312 operates properly. When the vehicle performs a
self-testing, a testing signal is applied to the power line 306 for
a certain time period, for example, 5 seconds. In one embodiment,
the testing signal can be a square wave signal. A detecting circuit
320 can detect a voltage drop across the circuit 300. If the
waveform of the testing signal is detected across the circuit 300,
the detecting circuit 320 can determine that the light source fails
to be turned on and can generate an alarm signal to turning on an
indicator light on the dashboard. If the waveform of the testing
signal is not detected across the circuit 300, the detecting
circuit 320 may determine that the light source operates properly,
e.g., is turned on successfully.
In operation, the monitoring unit 302 can detect the testing signal
and can turn on the switch 318 to conduct the current path in
response to the testing signal. A detailed structure of the
monitoring unit 302 according to one embodiment of present
invention is illustrated in FIG. 4. When the testing signal is
detected, a current is enabled to flow through the dummy load 304
to ground. In other words, the dummy load 304 is parallelly coupled
to the circuit 300 when the switch 318 is turned on. As a result,
the total resistance of the circuit 300 is decreased. By choosing a
dummy load 304 with a proper resistance, the total resistance of
the circuit 300 can be reduced. Accordingly, the amplitude of the
waveform of the testing signal can be small enough that the
detecting circuit 320 does not detect the waveform of the testing
signal across the circuit 300. Thus, a false alarm can be avoided.
When the self-testing is completed (the testing signal is absent
from the power line 306), the monitoring unit 302 can turn off the
switch 318 to cut off the current path, so that the dummy load 304
no longer consumes power.
In addition, the controller 308 can detect if there is an abnormal
or undesired condition of the LED string 312 according to the
current flowing through the LED string 312. For example, if the LED
string 312 is in an open circuit condition, the current flowing
through the LED string 312 can be substantially zero which is less
than a first predetermined current level. If the LED string 312 is
in a short circuit condition, the current flowing through the LED
string 312 can be greater than a second predetermined current
level. Therefore, abnormal/undesired conditions of the LED string
312 such as open circuit and short circuit conditions can be
detected by the controller 308 by comparing the current flowing
through the LED string 312 with one or more predetermined current
references. In one embodiment, the controller 308 can turn off the
switch 318 to cut off the current path if an abnormal/undesired
condition is detected. As a result, the detecting circuit 320 is
able to detect the waveform of the testing signal and generate an
alarm signal. Moreover, the controller 308 can control the DC/DC
converter 310 based on the current flowing through the LED string
312 such that the DC/DC converter 310 can provide regulated power
to the LED string 312. Thus, the LED string 312 can have a desired
brightness.
FIG. 4 shows the circuit 300 using LEDs to replace an incandescent
bulb in a vehicle. A detailed structure of the monitoring unit 302
in FIG. 3 in accordance with one embodiment of the present
invention is illustrated in FIG. 4. Elements labeled the same as in
FIG. 3 have similar functions.
In the example of FIG. 4, the dummy load 304 includes a resistor
416, the switch 318 includes a transistor 412, and the current
sensor 316 includes a resistor 414. In one embodiment, the
monitoring unit 302 can include a first capacitor 402 coupled to
the power line 306, a first diode 404 with a cathode coupled to the
first capacitor 402 and an anode coupled to ground, a second diode
406 with an anode coupled to the first capacitor 402 and a cathode
coupled to ground through a second capacitor 410, and a resistor
408 coupled in parallel with the second capacitor 410. A gate
terminal of the transistor 412 is coupled to the second capacitor
410 such that an on/off status of the transistor 412 can be
determined by a voltage across the second capacitor 410. In one
embodiment, the transistor 412 has a threshold voltage Vth. If the
voltage V410 across the second capacitor 410 is less than Vth, the
transistor 412 is turned off. If the voltage V410 across the second
capacitor 410 is greater than Vth, the transistor 412 is turned on.
FIG. 5 illustrates a relationship between the voltage V410 across
the capacitor 410 and the testing signal applied on the power line
306, in accordance with one embodiment of the present invention.
FIG. 4 is described in combination with FIG. 5.
In operation, if the vehicle does not perform a self-testing, there
is no testing signal applied on the power line 306. The power line
306 can provide DC power, e.g., a 12V DC voltage, to the circuit
300. The DC power is isolated by the first capacitor 402 such that
the second capacitor 410 is not charged. The voltage V410 across
the second capacitor 410 is less than the threshold Vth. As a
result, the transistor 412 is turned off and the current path is
cut off.
In operation, if the vehicle performs a self-testing, a testing
signal (shown in FIG. 5) is applied on the power line 306. Since
the testing signal has an AC (alternating current) component, the
testing signal can pass through the first capacitor 402 and the
second diode 406 to charge the second capacitor 410. As can be seen
in FIG. 5, when the testing signal is high, the second capacitor
410 is charged such that V410 increases. When the testing signal is
low, the capacitor 410 is discharged through the resistor 408 such
that V410 decreases.
There can be several ways to choose the proper capacitance of the
first capacitor 402, the capacitance of the second capacitor 410,
and the resistance of the resistor 408. In one embodiment, the
capacitance of the first capacitor 402, the capacitance of the
second capacitor 410, and the resistance of the resistor 408 can be
properly chosen such that the voltage V410 across the second
capacitor 410 reaches a dynamic balance state after a number of
consecutive pulses of the testing signal are applied to the power
line 306. In one embodiment, the dynamic balance state is obtained
when the average value of V410 becomes substantially constant. In
one embodiment, the greater the capacitance of the second capacitor
410 and the resistance of the resistor 408, the slower the
discharge process of the second capacitor 410 will be. For a
testing signal having a lower frequency, the second capacitor 410
has a slower discharge process such that the voltage V410 across
the second capacitor 410 can reach the dynamic balance state after
a number of consecutive pulses of the testing signal are applied to
the power line 306.
In another embodiment, the capacitance of the first capacitor 402
and the capacitance of the second capacitor 410 can be properly
chosen such that V410 is less than the threshold voltage of the
transistor 412 when there is no testing signal asserted to the
power line 306. Thus, the transistor 412 is turned off. Assuming
that the DC voltage provided by the power line 306 is V.sub.DD, the
capacitance of the first capacitor 402 is C.sub.402, the
capacitance of the second capacitor 410 is C.sub.410, the voltage
across the second diode 406 is V.sub.D2, the threshold voltage of
the transistor 412 is Vth, C.sub.402 and C.sub.410 can be
determined according to formula (1) and formula (2).
.times..times.< ##EQU00001##
FIG. 6 shows a flowchart 600 of a method for powering a light
source in a vehicle, in accordance with one embodiment of the
present invention. FIG. 6 is described in combination with FIG. 3
and FIG. 4.
In block 602, a testing signal on a power line 306 is monitored,
for example, by a monitoring unit 302 in a circuit 300.
In block 604, a current path is conducted to enable a current
flowing though a dummy load 304 to decrease a total resistance of
the circuit 300. In one embodiment, a capacitor 410 is charged by
the testing signal. A voltage across the capacitor 410 is greater
than a threshold of a switch 318 such that the switch 318 can be
turned on to conduct the current path. As a result, the dummy load
304 is parallelly coupled to the circuit 300 and therefore the
total resistance of the circuit 300 can be decreased. Accordingly,
the waveform of the testing signal is not detected across the
circuit 300 by a detecting circuit 320. Thus, a false alarm can be
avoided.
In block 606, if the testing signal is absent from the power line
306, the current path is cut off. In one embodiment, when the
testing signal is absent, the voltage across the capacitor 410 is
less than the threshold of the switch 318. Thus, the switch 318 can
be turned off, and the current path is cut off. Therefore, the
dummy load 304 no longer consumes power.
Accordingly, embodiments in accordance with the present invention
provide a circuit for driving a vehicle lamp. The circuit can use
an LED string as a light source to replace an incandescent bulb in
the vehicle lamp. Advantageously, when the vehicle performs a
self-testing to examine if the vehicle lamp operates properly, the
circuit can conduct a current path to prevent a false alarm from
being triggered. Furthermore, by cutting off the current path when
the vehicle does not perform a self-testing, the power can be saved
and heat dissipation can be reduced.
While the foregoing description and drawings represent embodiments
of the present invention, it will be understood that various
additions, modifications and substitutions may be made therein
without departing from the spirit and scope of the principles of
the present invention as defined in the accompanying claims. One
skilled in the art will appreciate that the invention may be used
with many modifications of form, structure, arrangement,
proportions, materials, elements, and components and otherwise,
used in the practice of the invention, which are particularly
adapted to specific environments and operative requirements without
departing from the principles of the present invention. The
presently disclosed embodiments are therefore to be considered in
all respects as illustrative and not restrictive, the scope of the
invention being indicated by the appended claims and their legal
equivalents, and not limited to the foregoing description
* * * * *